1. Field of the Invention
This invention relates to a disc-shaped recording medium for recording a variety of information signals, such as speech or video signals.
2. Description of Related Art
Among the recording mediums for recording information signals, such as that for audio or video, there are known disc-shaped optical recording medium and a disc-shaped magnetic recording medium.
Among these recording mediums, there are an optical disc, on which information signals are written as micro-irregularities, such as by pits and grooves, a phase-change optical disc, a magneto-optical disc, exploiting photomagnetic effects of the recording film, and a hard disc for magnetically writing signals.
For forming a recording layer on an optical recording medium, among these recording mediums, having micro-irregularities, such as phase pits or pre-grooves, in association with information signals, such as data information or tracking servo signals, injection molding of a substrate of plastics material is routinely employed. Specifically, a disc-shaped substrate is formed using an injection molding device, a metal mold and a stamper, and information signals are transcribed at this time from the stamper.
Meanwhile, in a substrate of a conventional disc-shaped recording medium, such as a so-called digital audio disc (CD) or a digital versatile disc (DVD), referred to below as a disc substrate, in which the reproducing light is illuminated to pass through the bulk of the disc substrate to read out the information signals, the disc substrate is required to exhibit optical transparent characteristics and to be freed of birefringence to the maximum extent possible. In addition, the molding is required to be carried out with microscopic signal transfer characteristics.
In addition, since a flight head is used for recording and/or reproduction, a hard disc substrate is required to exhibit planarity close to that of glass, and hence it is necessary to use resin of high fluidity capable of transcribing signals in a state free from micro-protrusions.
For producing this disc substrate by injection molding, the crucial point is how to control a variety of internal stresses produced in the respective molding steps, including resin molding, injection charging into a metal mold or cooling for solidification, such as friction with the metal mold when the resin is injected and molded into the metal mold, a variety of stresses, pressure, temperature etc produced at the time of injection, or a variety of mechanical properties, such as defective transciption in the disc substrate, birefringence, deformation due to warping, or the like.
However, even if the internal loss of the substrate is increased to improve resonance characteristics and toughness of the disc substrate itself, it is difficult with the disc substrate of the conventional structure comprised of the uncompounded polymer to realize these properties simultaneously, since these properties are innate to the polymeric structure proper to the resin and cannot be coped with by the molding device or technique, and also because the high optical properties, large internal loss and high stiffness of the disc substrate are contradictory to one another.
With the tendency to higher recording density and especially to higher numerical aperture (NA), the light from the objective lens is throttled more severely than in the conventional CD or DVD, with the result that the recording and/or reproducing signal location is closer to the disc surface than to the disc bottom, with the tolerance for the warping or undulations of the disc substrate being decreased significantly.
This is evident from the following relation between the NA of the objective lens and the disc thickness and between the wavelength and NA:f=D/2NA>WD                f: focal length of the lens        D: effective diameter of the objective lens;        NA: numerical aperture of the objective lens;        WD: operating distance of the objective lensdepth of focus=λ/(NA)2skew tolerance∝λ/(NA)3 thickness variation tolerance∝λ/(NA)4  disc toughness∝(thickness)3        
From the above equation, the relation between NA and the disc thickness as the light transmitting layer may be found as follows:For NA=0.5, the disc thickness=1.2 mm (CD)For NA=0.6, the disc thickness=0.6 mm (DVD)For NA=0.75, the disc thickness=0.3 mmFor NA=0.85, the disc thickness=0.1 mm.
In addition, there is a relation that the disc toughness is proportionate to the third power of the thickness, such that, as the disc becomes thinner in thickness with an increasing recording density, the disc becomes inferior in mechanical properties, such as bending strength. Moreover, under environment of heat, film stress, temperature or humidity, applied at the time of molding, the bimetallic deformation due to moisture absorption is liable to occur significantly in an optical recording medium adapted for recording on only one side, with the result that use of an optically transparent disc is virtually impossible.
Among the methods for molding a high NA high density disc substrate, there are a substrate molding method by two-color concurrent molding and a two-color core-back or core-rotating molding method employing a molding device having two injection units for molding two-layer structure or three-layer sandwich structure, with the use of a conventional optical resin having good fluidity for a skin layer and a resin having improved resonance characteristics or stiffness for a core layer.
However, the disc substrate having good signal transcription characteristics and stiffness cannot be realized with the use solely of a molding method or a molding device, since these properties are innate to the resin or to the resin composition. Thus, there is presented a demand for a resin composition satisfying these contrary characteristics, specifically the resin composition having good signal transcription properties and high stiffness.